Mother Slime

Killing one does not end the encounter. It bursts into a handful of baby slimes on the spot, so in a one tile passage you have traded a single enemy for three bouncing ones with nowhere to stand. Explosives answer both halves in one go, since the blast that finishes the parent usually catches the children before they scatter. A Grenade is plenty at the point in a run where you meet these.

It is among the biggest slimes you will meet outside of a boss fight and it is still hard to see, because the color sits right on top of the unlit cavern background. Often the first sign of one is the damage number. That matters most when you are tunneling with a mining loadout and no defense on. Light the ceiling ahead of you and glance above ledges before dropping down. Royal Gel turns the whole problem off, since it goes passive like every other slime, and their spawn rate collapses once the world flips to Hardmode, so the window in which any of this matters is short.